1. Field of the Invention
The present invention relates to a monostable circuit as a semiconductor circuit and, in particular, a retriggerable multivibrator (retriggerable monostable circuit).
2. Description of the Related Art
As an example of a conventional retriggerable multivibrator of this type is the Toshiba product type "TC74HC4538P/F" as shown in FIG. 1, with associated operation waveforms shown in FIG. 2. In the circuit of FIG. 1, C and R represent a capacitor and resistor, respectively, which are connected external to an integrated circuit. The conventional retriggerable multivibrator has an input A triggered by a rise edge of an input and an input 8 triggered by a fall edge of an input and complementary outputs Q, Q. Output Q, an output Q goes high when the input A rises when the input B is at a high level is and the input CDH at a high level H. In this case, an internal switch connected to node T2 is turned ON, lowering a potential on the node T2 (releasing a charge on capacitor C), which has heretofore been held at a power source voltage Vcc level. The internal switch is turned OFF when the potential on the node T2 reaches a low level (L) reference input potential Vref. At this time, a charging current flows from the Vcc node into a circuit of a resistor R and capacitor C and the potential on the node T2 rises due to the time constant of the aforementioned circuit. When the potential on the node T2 reaches a high level reference input potential Vref (H), the output Q is reset to a low level. That is, the output Q delivers a pulse of a predetermined width TW which is determined by the aforementioned capacitor C and resistor R.
When the input A rises during a period of time in which the output Q is in the high level (prior to being reset to the low level), then the internal switch is so controlled as to again charge the node T2 at that time. In this case, the pulse width TW of the output Q is extended as to correspond to TW+tr where tr is equal to a time from the first input A to the second input A. Upon the entry of the second input A, the preceding first input A is disregarded and the high level of the output Q continues over the time period TW from the time of a reentry of the input signal. Thus when the input A enters in a continuous fashion at a shorter period of time than the time period TW, the output Q is held at a high level.
The aforementioned conventional retriggerable multivibrator has its output pulse width TW controlled by the capacitor C and resistor R provided external to the integral circuit, but in spite of this general nature a greater number of component parts need to be mounted on a printed circuit board. This presents a problem of requiring a greater mounting area, more manufacturing steps, etc. Furthermore, the retriggerable multivibrator per se requires two voltage comparators for comparing the voltage on the node T2 with the reference input voltage Vref (L) or Vref (H), further complicating the circuit layout.